1. Field of the Invention
This invention relates generally to laser light sources and more particularly to laser light radiation sources for precision measurements, having extremely small beam diameters with Gaussian intensity distribution and circular shape. These lasers generally use solid state diode laser sources in combination with optical elements for specific applications including optical disk recording and laser printing.
2. Description of the Prior Art
A diode laser is very small compared with a gas laser, but it emits high optical power. The beam is divergent. The divergence is different in two directions. When it is collimated, the beam is oval shape and the spot focused is elongated. To obtain a small, high energy, circular spot the beam must be made round in shape. This is done using two prisms as described in U.S. Pat. No. 4,609,258 to Adachi et.al. The application of such a collimated beam is for optical disk recording with the beam diameter 4 mm, or for laser printers, a beam diameter of 8 mm. There is need for a smaller diameter beam with strong intensity for applications as an optical "pointer" for alignmnent, and robotics vision (line illumination). In order to obtain a small diameter beam, in accordance with the prior art U.S. Pat. No. '258, a diverging beam is collimated by a lens and the oval shaped beam is corrected by a beam shaper consisting of two prisms, obtaining a 4 mm beam diameter, then demagnifying the beam diameter using an inverse Gallileo type telescope as shown in FIG. 1.
In order to make this system work:
1. The beam before the demagnifier must be well collimated
2. The air space adjustment of the demagnifier is very critical and must be carefully made.
At those steps, the output collimated beam qualities are tested by a Radial Shearing Interferometer as shown in FIG. 2. The analyzing interferogram is adjusted to the best wavefront aberration. The spot at the 50 ft test distance is compared. The flare makes a soft spot and a very difficult to obtain 1 mm sharp spot.
See: D. Malakara: Optical shop testing P. 145, Radial Initial Caps John Wiley and Sons 1977
Another method to collimate the beam is to use a graded index rod lens (Selfoc). This method loses the energy almost 35% because the numerical aperture NA of the Selfoc lens is small (=0.3) The 3 mm diameter Selfco lens has to be stopped to 2 mm to avoid any variation in refractive index distribution. The residual spherical aberration is 0.25, wave peak to valley. This has to adjusted by focus at the defined distance. Because the beam is not shaped, the spot is oval shaped.